DIFFUSION, in physics, is the gradual and spontaneous molecular intermingling of two gases or two liquids in contact into a homo geneous mixture. It takes place regardless of or in opposition to gravitation and is due to the mutual attraction of molecules. Diffusion is most commonly observed between gases, and the lighter the gas the faster does diffusion take place. It also occurs between viscous substances and, in some cases, between solids. A common experiment, exhibiting the diffusion of gases, is to place mouth to mouth vessels of hydrogen and oxygen. After a time it is found that the con tents of both vessels are alike — a uniform mixture of the two gases. If the heavier gas is placed below the lighter the result is the same, demonstrating the fact that gravity does not check the result. If chlorine is one of the gases used its color will enable the eye to follow the diffusion. If a porous body is placed between the vessels the rate of diffusion can be measured. This fact is taken advantage of in the construc tion of the diffusiometer, invented by Thomas Graham. This instrument consists essentially of a tube containing a gas, the lower and open end of the tube being dipped in mercury, while the upper end is closed with a porous plug. The rise of the mercury in the tube serves to measure the diffusion of the gas through the plug. Another method of testing diffusion is to fill a diffusion tube with for instance hydrogen — and im merse the lower and open end in water, while the upper end is closed with the porous plug. Diffusion then takes place both ways, into the water and into the air. • Alcohol and water afford one of the readiest means of testing the diffusion of liquids. They will diffuse the same as gases, regardless of gravity. Stirring the two hastens the diffusion, because it brings a larger surface of one liquid against the other. At the same time the action is not a mixing in the ordinary sense, but a mix ing of the molecules, forming a liquid that is of like composition throughout when the diffusion is complete. If water in a glass vessel be colored with a few drops of litmus solution and sul phuric acid be introduced through a tube to the bottom of the vessel, care being taken not to disturb the water, the diffusion may be observed visually. The water, which is blued by the
litmus, turns from blue to red as the diffusion of the sulphuric acid progresses. The principle of diffusion is used in sugar making for extracting sugar from cane-juice and also from beets. Hot water is applied to the cut cane and sliced beets and diffusion withdraws a large part of the sugar, leaving in place the troublesome colloid matters which are present in the pressed juice of both beets and cane. Surgeons have also utilized diffusion to cause a liquid to pass through a membrane or tissue of the body. It is the principle of diffusion of liquids that en ables the druggist to compound the several items of a prescription. The uniform strength of his solutions and extracts is also largely dependent upon diffusion. If it were not for this several fluids in a vessel would tend to arrange them selves in layers according to their gravity, as is the case with water and oil, which do not dif fuse. Substances in solution also diffuse, following Fick's law of the diffusion of heat, that the diffusion of substances in solution is comparable to the travel of heat in conductors. As the temperature rises the rate of diffusion in creases. Since the rate of diffusion of different solutions and liquids varies, it becomes possible to separate component substances in solution by taking advantage of the difference in rate of diffusion of each component. It has been demonstrated that some solids diffuse, though with extreme slowness. Lead placed upon gold for a period of years will be found to contain gold to a slight distance above the point of con tact. When the metals are kept at a tempera ture of from 300° .to 400° F., a recognizable diffusion takes place within 30 or 40 days.
For a fuller understanding of the diffusion of gases, see KINETIC THEORY. Compare also smosis, which is, practically, diffusion taking place through a membrane.